Increasing use is being found in industrial applications for photopolymerizable coatings. By "photopolymerizable coating" we mean a coating placed upon a substrate in which the curing step to form the final coating firmly affixed to the substrate is initiated by exposure of the coating to some form of radiation. In typical applications, exposure to ultraviolet ("uv") radiation is the preferred means of curing. Typical uv radiation is sufficiently energetic to be a useful initiator of certain chemical reactions when a suitable photoinitiating chemical species is present. One of the advantages of using uv radiation is that, for typical wavelengths used for photocurable coatings, these wavelengths are not commonly present as a significant component of background ambient radiation to which the coating may be exposed during handling or storage incidental to the coating process, or during subsequent use of the coated article (tending to cause discoloration or hazing of the coating). Among the disadvantages of uv radiation as a curing agent are the relative expense and safety considerations in the production and use of uv. Therefore, as an alternative to uv curable coatings, certain visible wavelengths may also provide initiation of photopolymerization when used in conjunction with the appropriate light-absorbing chemical species. Visible light as a photoinitiator suffers from the disadvantages noted above related to accidental or incidental exposure, but also suffers from the disadvantage of being much less energetic, photon for photon, than uv. This requires the use of different chemistry, typically related to dyes or other molecules possessing relatively large regions of conjugated unsaturations for purposes of lowering the typical absorption energies of the molecule from the uv to visible spectral regions.
In addition, electromagnetic radiation more energetic than uv (such as x-ray) may provide initiation of photopolymerization. Also, exposure to electron beams may be used as the means to initiate polymerization of the coating and, hence, cure. One of the advantages of using electron beams is the fact that the incident electron is typically sufficiently energetic to initiate the reaction directly without the intervention of special photoinitiating species. The oligomers of the coating to be cured directly interact with the incident electron beam to form the species necessary to react further without the need for photoinitiators to be added to the mixture. However, typically such process must be carried out in a vacuum as the electron beam does not significantly penetrate air, thus adding to the complexity of such processes for industrial production applications. Electron beam equipment is typically very costly compared to other forms of photoinitiating sources of radiation.
We distinguish herein "radiation curing" from "thermal curing". By "radiation curing" we intend to mean the interaction of a molecule with electromagnetic or other radiation (commonly electron beams) in which a specific molecular energy state, or set of states, are caused to be excited, resulting in curing of the resin. The excitation of molecular energy states by such radiation results in a population of molecular energy levels different from the normal Boltzmann population distribution expected from the application of heat in "thermal curing". Confusion may occur when considering the various methods in which heat may be applied to a coating to effect thermal cure. Ovens or infra-red lamps are the typical sources of heat for thermal cures. Both of these energy sources are not typically suited for effecting radiation curing as defined herein. Thus, thermal curing is the obvious mechanism intended. However, some researchers (Holliday, UK patent 2,056,885) use uv radiation as a source of heat, not excitation radiation for specific energy levels. In this case, care must be taken to note the specific chemical reactions following the exposure to uv radiation. If such reactions are typical of thermal curing, then uv radiation is used for a source of heat, not level-specific molecular excitations. Herein we use "thermal curing" to mean curing by means of heat and Boltzmann distributions of molecular energy levels, even if that heat may be supplied by means of exposure to uv radiation.
For the purposes of the present invention we will use the term "ultraviolet" or "uv" to denote radiation which is used to initiate photopolymerization other than by thermal means; that is "radiation curing." We specifically exclude from this definition use of uv as a source of heat (see Holliday) with the understanding that not all exposures to uv radiation effect "radiation curing". The invention described herein is not intended to be limited thereby to exclude exposure to other forms of radiation, such as electron beams or radiation more or less energetic than uv (typically x-rays or visible radiation respectively), which in specific instances initiate photopolymerization (as distinguished from thermal curing), unless specifically stated in conjunction with the particular compounds or chemistry under consideration. Since ultraviolet is expected to be the preferred radiation for the practice of the chemistry of the present invention, we will use the term "uv" for economy of language.
There are several reasons for the increasing interest in uv curable coatings. Typically, uv curable coatings are applied to the substrate to be coated without significant amounts of solvent present in the formulation. Commonly, uv curable coatings applied in the liquid form are delivered in a mixture of monomers, which polymerize rather than evaporate in the subsequent uv cure. Therefore, the curing step involves polymerization of the coating without significant drying. Hence, insignificant (or greatly reduced) amounts of emissions into the air are present when uv curable coatings are used. Environmental concerns for exposure of the workers engaged in the coating process, as well as the general population concerned with improving air quality, are thereby served. While reducing solvent emissions into the air is an advantage of liquid uv curable coatings, the powder uv curable coatings described herein typically reduce emissions even further.
Typical uv curable coatings employ less energy than thermal cures and typically require significantly less cure time. Reduction of the energy required for the curing of the coating clearly is advantageous in reducing process costs. Reduction of curing time reduces the quantity of work-in-progress for a given rate of production. This results in savings in terms of the costs associated with the product flowing through the coating process. Reduced curing time also allows the coating industry to respond more rapidly to customer preferences and specific orders, much in keeping with the modern trend to "just in time" manufacturing.
Additionally, less factory floor space is typically required for uv curing systems than is commonly the case for other means of curing, leading to cost reduction by lowering the investment required in building space and the expenses of building maintenance, utilities, etc.
It is commonly the case for uv curable coatings that additional coatings may be applied immediately following the uv cure. Such immediate recoatability is not typically the case for most conventional coating and curing processes.
A significant advantage of uv curable coatings (in contrast to thermally cured coatings) lies in the avoidance of heating of the substrate to elevated temperatures in order to effect cure. The present invention was motivated in part by the need to coat wood, wood-related products, or other heat-sensitive substrates. UV curable coatings offer a very substantial advantage in requiring much less input of heat and, therefore, much lower surface temperatures, allowing such heat-sensitive substrates to be coated and cured without heat-induced damage.
The general field of photochemistry, and the particular field of photoinitiated polymerization, is a large and active one with an extensive literature and significant continuing research. In contrast to the present invention, virtually none of this prior research involves the use of solid reactants for coatings. In order to fix terminology, we review here some of the basic concepts of photopolymerization.
The initial step in photopolymerization reactions is the transfer of energy from the incident radiation (electromagnetic or otherwise) to the chemical system to be photopolymerized. There are two general categories of interaction mechanism by which this may occur.
In one instance, the incident radiation is absorbed by an absorbing molecule within the chemical system, but the absorbing molecule is not itself consumed in subsequent chemical reactions. In this case, the absorbing molecule is generally termed a "photosensitizer" since it acts as a photocatalyst; promoting chemical reactions without itself being consumed. The incident radiation in this case results in the excitation of internal energy states of the photosensitizer. Typically, the internal excitations will involve electronic excitations of low lying electronic states of the photosensitizer, but other modes of internal excitation are also possible (such as vibrational, rotational or combinations of electronic with vibrational and rotational modes). The excited photosensitizer then undergoes intermolecular energy transfer, transferring its internal energy thereby to another molecule which is the initiator of the polymerization reaction. By means of this intermolecular energy transfer, the photosensitizer is returned to its original state (typically the ground state), ready for further excitation by incident radiation.
However, strict photocatalysis (in which all of the photoabsorber is regenerated for additional uv absorption) has some disadvantages when used for the curing of coatings on surfaces. Typically, except for very thin coatings, the incident curing radiation will not penetrate effectively throughout the entire thickness of the coating, leading to faster or more complete curing on the surface than in lower layers. It is, therefore, useful for the photoinitiator to become slowly ineffective at absorbing radiation, permitting greater penetration of the curing radiation through the upper levels of the coating to reach (and cure) the lower levels. Such "photobleaching" must occur at a rate slow enough to permit complete curing of the upper surface before the photoinitiating species is destroyed, but rapid enough to allow curing of lower layers in reasonable times. Such photobleaching chemicals and mixtures are the subject of active research primarily directed to the field of liquid photopolymerization systems, and are likely to become increasingly useful for thicker coatings in which penetration of radiation to lower levels is more difficult.
Another general category of initiation mechanism for photopolymerization reactions is the case in which the radiation is absorbed by the molecule which begins the first step of the polymerization reaction. That is, the radiation absorber is consumed in the reaction as the chain initiator for subsequent polymerization. This is generally referred to as "photoinitiator". Typically, the photoinitiator will be excited, fragmented or ionized by means of its interaction with the incident radiation. The resulting species initiate polymerization and are consumed by the reaction. Thus, radiation must be absorbed by a supply of photoinitiators, continuing until the photoinitiators are consumed.
Photocatalysis (via a photosensitizer) and photoinitiation (as defined herein) are very different conceptual chemical processes: one leading to consumption of the radiation-absorbing species, one not. However, the actual terminology as commonly used in the field is not so precise. "Photoinitiator" is generally used to mean any species which interacts with the incident radiation, whether consumed in the reaction or not. Thus, common usage has photosensitizer as a subclass of the class of photoinitiators. The present invention does not critically distinguish between the use of photosensitizers and the use of (noncatalytic) photoinitiators. Therefore, we will adopt the common usage herein and use "photoinitiator" to encompass both catalytic photosensitizers and consumable photoinitiators. Specific distinctions will be made where necessary.
Our prior description is directed at the most common case of photoinitiation. That is, incident radiation is absorbed by a molecule leading to excitation and intermolecular energy transfer (in the case of photosensitizers), or excitation, fragmentation or ionization followed by initiation of polymerization (in the case of non-catalytic photoinitiators). However, other mechanisms are possible. The absorbing species need not be a separate or distinct molecule, but may be a specific functional group contained as a portion of a distinct molecule residing in such a chemical environment that the functional group may be treated as a nearly-distinct species for purposes of interaction with incident radiation. Also, the specific functional group may itself interact with the incident radiation, or it may alter the characteristics of its host molecule leading to absorption of radiation directly by the host (which itself is thereby the photoinitiator). For purposes of the present invention, we treat as "photoinitiation" any such mechanism of interaction with incident radiation.
In the above discussion we referred to "absorption" of the incident radiation. While this is perhaps the predominant mechanism in use for photopolymerization, the incident radiation need not be fully absorbed by the photoinitiating species. It is merely necessary that the incident radiation transfer energy to the photoinitiating molecule and thereby initiate subsequent polymerization. The incident radiation may be fully absorbed in the process of transferring energy, or it may simply be scattered by the photoinitiator, leaving some energy behind ("inelastic" or "Raman scattering"). Absorption is probably the most common mechanism when uv or other electromagnetic radiation is used for photopolymerization (although inelastic scattering is not ruled out). However, when electron or other particle beams are used to initiate photopolymerization, inelastic scattering tends to predominate over absorption. That is, the incident beam of electrons is more likely to initiate photopolymerization by means of electromagnetic energy transfer as the charged electron flies by the photoinitiating molecule, rather than becoming absorbed to create a negative ion. However, electron capture to form a negative ion is not ruled out in all cases. In discussions of the present invention, we will loosely use "absorption of radiation", "photoinitiation" and like terms to denote any, all, or any combination of photoinitiation mechanisms such as described herein or known in the field.
For purposes of discussion of the present invention, we will be primarily concerned with free radical curing systems. That is generally used to mean uv curable systems in which the primary photoinitiation step (and/or the polymer growth, propagation step) involves free radical chemical species. Free radical curing uv systems are perhaps the most commonly used in industrial applications at the present time. However, those currently in use are virtually exclusively liquid coatings. The common types of such coatings presently in use are generally acrylates (typically epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates), acrylated oils, unsaturated polyesters, vinyl/acrylics, or polyene/thiol systems.
The use of uv curable coatings may (but need not) involve two conceptually distinct polymerization reactions. The coating formulation may consist of monomers, oligomers, or a polymer which have remaining functionality which will typically be reacted (polymerized) in the curing step. After being applied to the substrate, the subsequent curing step involves polymerization of the coating to form the final cured coating on the substrate. If the coating is applied in the form of monomers, the curing polymerization is the only polymerization reaction carried out in the process. This is most common for liquid uv curable coatings. However, it is frequently the case for liquid as well as for solid coatings that, prior to application to the substrate and subsequent curing, the coating itself has been partially polymerized. Thus, the coating applied in this form is known as "oligomer", "prepolymer" or (typically for solid, powder coatings) a "resin". The curing then causes remaining functionality to react in forming the final coating. Therefore, coating materials and formulations prior to curing may be designated as polymers along with specific functionality (i.e. "polyester acrylates"). It is understood thereby that previously polymerized materials are applied to the substrate and the remaining functionality is reacted during curing. For the solid or powder coatings which are the subject of the present invention, the coatings will almost never be applied in the form of a monomer.
The discussion thus far of polymerization and curing upon exposure to uv radiation has been primarily derived from liquid systems used for the formation of uv curable coatings. It is a primary purpose of the present invention to describe formulations of uv curable coatings suitable for application to the substrate in the form of a powder. The application of a coating in the form of a powder eliminates all use of solvents or liquid monomers, substantially eliminating emissions to the air.
Powder coating technology has found wide usage in the application of coatings to numerous articles of commerce. It has been especially important in the coating of office furnishings, household appliances, shelving, yard furnishings, garden tractors and other equipment, certain automotive parts, and numerous other products. There are several reasons for the increasing utility of powder coating. Powder coating technology typically does not use solvents which otherwise provide a health hazard for the workers exposed to such solvents throughout the working day. Solvent emissions are increasingly an air pollution concern as well. Such solvents additionally cause serious environmental problems in planning for and managing their disposal as waste following the coating process. The absence of solvents as an item of expense also leads to cost savings for most powder coating processes.
UV curable coatings applied as a liquid typically are deposited on the substrate in a solution of monomers. Monomer liquids have many of the same drawbacks as do solvents in contributing to environmental pollution and increasing hazards to the health of the workers. Therefore, the present invention is directed at the use of uv curable powder coatings, eliminating thereby both solvent and monomer emissions into the environment.
Typical powder coatings can be collected for reuse, thereby reducing the amount of waste materials. Reduction of waste reduces the cost of the process by reducing the cost of materials not effective in the production of the desired coating, as well as reducing the costs of disposing of waste in an environmentally acceptable manner.
The typical application method for powder coatings involves electrostatic deposition. The substrate (typically an electrical conductor) is connected to electrical ground. Powder coating is electrostatically charged, and thereby attracted to the substrate by electrostatic forces.
There are two general methods for causing the powder coating to become electrically charged; corona charging and tribocharging. The corona charging procedure produces a corona discharge in the surrounding air. The powder coating is propelled through the corona discharge region, acquiring thereby an electrostatic charge. The corona charging method typically produces more charge on the powder, thereby increasing the deposition on the substrate. However, the corona typically produces a greater mix of charged species in addition to the desired charged powder. In some applications the presence of free ions produced by the typical corona is not a serious disadvantage. However, for other applications in which uniformity of charge on the powder coating is a concern, or the presence of free ions otherwise interferes with the coating process, the corona charging method may not be the preferred technique.
The second method in common use for charging the powder coating for deposition involves tribocharging in which rubbing and frictional forces between the powder and another material causes an electrostatic charge to be created on the powder. In one common approach, the powder is passed through a long tube by rotation in a spiral configuration. The friction of the powder with the walls of the tube leads to the acquisition by the powder particle of an electrostatic charge. A typical tribocharging unit of this type would have numerous tubes collected into a spiral bundle configuration, typically at least one meter in length to insure adequate tribocharging of a sufficient fraction of powder particles. Introduction of the powder into one end of the bundle, followed by rotation causes charged powder particles to emerge from the other end. While this procedure is one method of tribocharging, other techniques involving reciprocating motion, etc, are, or soon will be, commercially available.
Tribocharging eliminates the need for high voltage coronas, thereby increasing the safety of the operator and tending to decrease processing costs. However, the key characteristic in the selection of tribocharging over corona charging is often the different deposition which may be achieved with tribocharging. It is believed that tribocharging tends to produce a more uniform charging of the powder. The lack of free ions and increased uniformity of charge causes tribocharging to yield superior coating properties for many powder coating applications. In addition, the charging of the powder coating is more easily controlled when tribocharging is employed. The length and material of the tribocharging bundle (for the typical spiral bundle tribocharging device) can be adjusted to allow for a wide range of charging characteristics of the powder to be applied. Similar ease of processes control may be envisioned for other tribocharging devices as they become available.
Once deposited on the substrate, the powder coating is cured to its final finish, typically in the prior art by means of heat, either by passage through a curing oven for the appropriate time or by irradiation by means of infra-red lamps of the required intensity and wavelength to which the workpiece is exposed for the required period of time. Occasionally (as noted above) uv or other sources of radiation may be employed as source of heat without utilizing the radiation curing potential of such energy sources.
Electrostatic deposition of typical powder coatings onto conducting substrates provides significant advantages in coating textured surfaces, or other small or hard-to-access regions of a workpiece. The electrostatic attraction of the grounded substrate in regions as yet uncoated by powder in preference to those regions having (typically insulating) powder coatings thereon, tends to attract powder to all regions of the workpiece.
Powder coatings typically involve a complex mixture of chemicals. In addition to the color-carrying pigment, a typical powder paint would have one or more resins, one or more curing agents, flow and leveling agents, degassing agents, waxes, extender pigments (fillers), and perhaps additional additives selected for particular applications and purposes. Typical powder coatings would also commonly be found to contain a charging agent to assist in the process of charging of the powder and the retention of charge for application onto the substrate to be coated.
Furthermore, agents may be added to powder coatings for the purpose of adjusting the viscosity to improve the smoothness and wetting characteristics of the applied coating. Such flow control agents are commercially available in a variety of formulations from a variety of vendors.
Agents may also be added to powder coating formulations to increase storage life by hindering thermosetting or radiation curing until deliberately undertaken in the curing of the coating. Such inhibitors may take the form of very small amounts of chemicals intended to suppress curing initiated by very small quantities of initiators accidentally induced. However, when deliberate curing is commenced, such cure-suppression agents would typically be completely overwhelmed by curing reactions and have no substantial effect on the curing process.
In addition to curing agents, typical powder coatings would also commonly be found to contain a charging agent to assist in the processing of charging of the powder and the retention of charge for application onto the substrate to be coated. The work of Macholdt et. al. (4,957,841 and 5,021,473) are examples of the use of such charging agents. As the work of Macholdt makes clear, charging agents are often useful in both corona discharge and triboelectric charging processes.
All of the above additives are in addition to the particular resins or other agents added by the powder coating manufacturer for the purpose of providing the coating color, texture, gloss, hardness, radiation and solvent resistance, etc. which typically are used to set apart one coating from another in the market.
The application of powder coatings to wood leads to additional challenges. For simplicity of language we will use the term "wood" herein to denote conventional wood in all varieties from all varieties of trees and shrubs (as such plants may be used in the form of chips, dust or powder), as well as products derived from wood such as plywood, particle board, fiber board, medium-density-fiber-board "MDF", and other wood-related products as would be typically used in commerce in a manner similar to wood, and for which a coating would be required or desirable.
Dry wood typically is made up of cellulose, lignin, various hemicellulose compounds, as well as numerous other components in lesser amounts, with considerable variation in relative composition from one type of wood to another. However, the water content of wood is a dominant component of non-baked wood and varies widely from one species of tree (and type of wood) to another and from one part of the same tree to another. Pine wood, for example, may have moisture content as low as 30% in heartwood to over 200% in sapwood (in which the % is defined as the weight of water divided by the weight of wood following oven drying.)
Perhaps the first problem which must be overcome in the development of powder coatings for wood is the problem of heating the wood. It is conventionally understood in working with wood that heating in excess of about 93 deg C. (200 deg. F.) is detrimental to the properties of the wood for further applications in furniture etc. Thus, in the application of powder coatings to wood, heating to temperatures above approximately 93 deg C. is to be avoided. This imposes considerable limitations on the formulations of powder coatings for use on wood and on the possibilities for using thermosetting resins, arguing strongly in favor of uv curable powder coatings, such as those of the present invention, for application to the coating of wood.
Typical wood and wood products also contain a certain amount of volatile components. Heating to any extent, even below 93 deg C., can cause such volatile materials to escape from the wood and introduce bubbles, pits or other unacceptable imperfections into the final coated product. Thus, it is desirable (although not necessarily essential) to minimize the heating of the wood during phases of the processing in which imperfections are likely to be introduced into the final coating.
Wood typically has a textured surface or a grain. This is not necessarily true of boards or other products derived from finely divided and compacted wood. But for many applications the textured surface of naturally-occurring wood is considered desirable by the consumer and may be artificially introduced into even those wood-derived products which could easily be manufactured with a high degree of smoothness. The typical wood texture of sanded or smoothed wood may not be obvious to human touch, but must be taken into account in designing a coating formulation and process. Even the small variations from ridges to valleys can cause coatings to miss certain areas of the surface or coat in various thicknesses, leading to unacceptable coating.
As noted above, electrostatic deposition of powder coatings onto conducting substrates provides significant advantages in coating textured surfaces since the electrostatic attraction of powder to the substrate tends to enhance coating of surface regions not as yet coated with (typically insulating) powder. Thus, in achieving a uniform powder coating of textured wood surfaces, electrostatic attraction may be expected to have certain advantages.
However, in the coating of typical woods, electrostatic attraction has not been widely employed. It is well known that dry wood (typically oven dried taken as fully dry wood) is a reasonably good electrical insulator but the resistivity can decrease by 13 or 14 orders of magnitude to lie in the range of 1,000 to 10,000 ohm-meters for wood having a moisture content at "fiber saturation". (See "Wood Engineering Handbook" by U.S. Forest Products Laboratory, 1974, pages 3-21 to 3-22.) Water can exist in wood bound chemically within cells and trapped in cavities. "Fiber saturation" is taken to mean a specimen of wood in which cells are saturated with all the water they can hold, but no water is trapped in cavities. Highly wet wood in which significant moisture is also trapped in cavities can have electrical resistivities reduced by another factor of 100 to the range of approximately 10 to 1,000 ohm-meters. However, the resistivity of wood having water trapped in cavities is highly variable from sample to sample even within the same type of wood processed in substantially the same way. Apparently, the exact nature of the cavities within the particular sample of wood, their geometry, interconnections, locations, density, etc. can have an significant and non-reproducible effect on measured electrical resistivity of various samples of wood.
The present invention provides a method for the electrostatic application of powder coatings to wood which leads to adequate covering properties while making use of the advantages of electrostatic coating technology. It is noted in the present invention that electrically grounding the sample of wood has a significant beneficial effect on the coating process, especially when used with tribocharging of the applied powder coating. Similar effects have also been noted by Holliday (UK Patent 2,056,885) in the context of electrostatic coatings of wood with thermally cured powders. However, the electrical conduction properties of wood are known to be sensitive to moisture content. It is also shown by the present invention that humidity control of the wood specimen before and during powder application can have an important effect in improving the coating properties of the powder.
In addition, moderate heating of the sample of wood is also demonstrated in the present invention to provide definite advantages in filling in textured regions of the wood surface and otherwise providing adequate coverage of all areas of the workpiece. Heating protocols leading to improved powder coatings, yet not exceeding heating guidelines generally considered acceptable for wood products, are also described. Specific formulations of powder coatings are also described in accordance with the present invention, making use of flow control agents (to control the temperatures at which the powder coating flow adequately into all regions of the wood to be coated).
Recently, the European Community has sponsored a study on the powder coating of MDF, wood and wood-related products under the SPRINT-Programme for Technology Transfer and Innovation (August 1993, Peter Svane). This SPRINT report describes uv curable powder coatings for MDF and wood-related products. However, the importance of the electrical properties of the wood or MDF workpiece in obtaining good powder coatings is not described. As a result, the effects of humidity control of the wooden substrate prior to powder application is not described, nor is the importance of electrically grounding the substrate during application. However, the present invention describes specific formulations of uv curable powder coatings distinct from, and leading to superior performance, from the formulations described in this SPRINT report.
The typical curing agents used in powder coating heretofore are thermosetting in which, under application of suitable heat from a bake oven or heating lamps, the curing agent causes the powder coating to harden into the desired finish. Epoxy and other heat-induced chemical reactions are typical means by which thermosetting curing occurs. However, as noted above, thermal curing of powder coatings offers several disadvantages. Certain substrate materials (such as wood or certain plastics) may not be heated above a certain temperature without causing harm to the material. This can place serious or insurmountable challenges in the way of thermal cures in striving to lower the cure temperature to an acceptable level while retaining the desired coating properties. UV curable coatings, such as the present invention, would be one way around thermal curing for heat-sensitive workpieces.
Also, certain workpieces are difficult to heat to the desired level due to the sheer bulk of the workpiece. For example in the application of powder coatings to certain motor vehicles is it not practical to heat the entire vehicle in its assembled state to the required curing temperature. Infra-red heating elements may ameliorate this problem somewhat. A uv curable powder coating would offer significant advantages in processing speed for large workpieces.
However, in the combination of uv curable coatings and powder coating technology additional complications and technical challenges arise. There are two basic grounds: 1) Low molecular mobility in solids, and 2) increased difficulty of light penetration through a solid for total curing of the entire film thickness ("through cure").
In a solid, such as a powder to be applied as a coating, molecular mobility and diffusion are essentially eliminated for all temperatures significantly below the flow temperature of the coating. Thus, in general, chemical species would need to be introduced into the powder formulation in sufficient concentrations and dispersions to lead to reactions in situ. Otherwise, chemical reactions for photocuring may proceed at slow, commercially unacceptable, rates thereby negating one of the major advantages to be gained from the use of uv curable coatings in the first place (that is, processing speed). This lower molecular mobility in solids places additional restrictions on the species, and combination of species, which may be used in powder coatings to be cured by uv radiation.
In addition, the use of powder coatings exacerbates the problems present in uv curing of thick liquid films. A primary difficulty in the uv curing of thick liquid films involves the penetration of the curing radiation throughout the entire thickness of the coating to give adequate "through cure." In the initial stages of curing, the incident radiation is typically absorbed by the photoinitiators present in the uppermost layers of coating. Two effects follow: 1) The upper levels of coating are fully cured well before the curing process is complete, and 2) The curing radiation is absorbed in the upper levels and thereby prevented from reaching the lower levels of the coating for adequate cure in these lower levels. Exposure to massive amounts of radiation to insure full penetration can lead to premature degradation of the properties of the cured upper layers (effectively accelerating the aging of the top levels of coating before the curing of the lower levels is even completed). This also has the disadvantage of increasing processing costs by requiring the generation and use of considerable radiation merely to have sufficient radiation survive the absorption in upper levels to achieve adequate through cure. Therefore, it is a serious problem in the choice of photoinitiators to insure adequate through cure without excessive exposure to radiation. The prospective use of self-bleaching photoinitiators is described above. The more typical approach, however, is to carefully tailor the formulation of photoinitiator and resin to balance absorption with penetration, and balance cure speed with economic use of radiation. In so far as solids typically present broader spectral absorptions than liquids, the problem of through curing is typically more challenging for powder coatings than for liquid coatings.
Additional technical problems arise when pigments are added to the coatings (both liquid and solid coatings). The curing radiation is typically uv or electron beam while the function of pigments is to provide color (that is, selective absorption and emission of electromagnetic radiation) in the visible portion of the spectrum. Thus, it is not inherently necessary that pigments interfere with the radiation curing of the coating since different portions of the electromagnetic spectrum are involved. However, in most practical pigmented systems in use today, the pigment provides an additional strong absorber of curing radiation, seriously complicating the uv curing of the coating. The conventional approach to the problem is to look for formulations and combinations of photoinitiators, pigments, and sources of curing radiation (typically uv lamps) which allow penetration of useful amounts of curing radiation through the pigment for absorption by the photoinitiator and photocuring of the resin (and pigmented additive therein).
It is generally true that the absorption spectrum of a molecule becomes more broad when in the solid state than when in either liquid or gas phase. Typically, the strong intermolecular forces associated with the formation of a solid will affect and broaden the absorption spectrum of a molecule. Thus, when overlapping and competing absorptions are a problem (as in the curing of thick, pigmented coatings), the problem tends to become worse in solids.
The present invention also considers the use of pigmented powder coatings, typically white or black coatings, in addition to clear coatings A choice of pigments is made so as to give adequate uv curing properties when used with certain photoinitiators, resins and appropriate sources of uv radiation. It is expected that colors (other than white or black) may also be produced by the methods of the present invention, modified only insofar as pigments recommended by the vendors for use with appropriate photoinitiators are substituted for the pigments discussed herein.
The prospective advantages of combining uv curable materials with powder coatings have attracted some previous work. In addition to the SPRINT work noted above, the invention of Iwase et. al. (U.S. Pat. No. 3,974,303 and UK Patent No. 1,446,119) describes a uv curable powder coating. This coating process involves heating to a molten state following deposition on the substrate material but prior to exposure to uv radiation (or possibly concurrently with the exposure to the curing uv radiation). This patent claims to avoid, or at least mitigate, some of the problems associated with simultaneous melting and thermal curing (such as bubble formation). Heating the coated workpiece to a temperature sufficient to cause the powder coating to flow and coalesce into a smooth film is necessary in virtually any powder coating process, including the process of Iwase, as well as in the present invention. However, it is important for many applications that the temperature be kept as low as possible. The Iwase process typically operates in the temperature range of 110.degree. to 120 deg. C. The present invention is an improvement on the Iwase process, and upon their formulation, in that specific formulations of uv curable resins (and other additives of the present invention) into a powder coating are described which lead in the present invention to superior performance in several aspects. In particular, unlike the work of Iwase, the present invention provides uv curable powder formulations capable of producing good surface finishes and surface properties at temperatures significantly below that of Iwase. As we discuss below, the present uv curable powder coatings are typically capable of flowing and coalescing at temperatures in the range 85.degree. to 100 deg. C. Certain materials suffer degradation in properties at temperatures above this level. Also, the use of lower temperatures provides savings in the energy required by the process irrespective of the properties of the substrate.
The work of McGinniss (U.S. Pat. Nos. 4,129,488 and 4,163,810) describes novel chemical compounds containing epoxy functionality bonded with ethylenically unsaturated polymers to form a compound, claimed by the inventor, to be suitable for uv curable powder coatings. The present invention does not involve a synthesis of specific and novel compounds for use as uv curable resins. Rather, the present invention comprises a formulation of known compounds, typically used for liquid coatings, but formulated and applied specifically for powder coatings.
The present invention relates to a formulation of compounds which leads to uv curable powder coatings without the necessity of synthesizing special compounds, as in the work of McGinniss, and leading to superior performance on certain substrates from the work of Iwase. The present invention demonstrates the formulation and use of uv curable powder coatings from chemicals typically available for use in liquid uv curable coatings. Thus, the formulation and processing of the present invention offers the possibility of uv curable powder coatings from reasonably available compounds and improved performance in certain applications on certain workpieces.